It seems Facebook is not satisfied with the third party Facebook desktop clients like Windows Live Messenger, because now they are testing their own client. The new client may not replace the web version of Facebook as a whole instead it will compliment the experience. It will offer multi-tab chat window, notifications, new news ticker stories, etc,. Obviously when you click any of the notification or stories on ticker it will open Facebook web in the browser.
You can check out here for more details about the client.